linux-kernel - Re: [PATCH v5 2/5] Bluetooth: hci_qca: Deassert RTS while baudrate change command

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Message-ID: <20190117172109.GU261387@google.com>
Date:   Thu, 17 Jan 2019 09:21:09 -0800
From:   Matthias Kaehlcke <mka@...omium.org>
To:     Johan Hovold <johan@...nel.org>
Cc:     Balakrishna Godavarthi <bgodavar@...eaurora.org>,
        marcel@...tmann.org, johan.hedberg@...il.com,
        linux-kernel@...r.kernel.org, linux-bluetooth@...r.kernel.org,
        hemantg@...eaurora.org, linux-arm-msm@...r.kernel.org,
        Johan Hovold <jhovold@...il.com>
Subject: Re: [PATCH v5 2/5] Bluetooth: hci_qca: Deassert RTS while baudrate
 change command

On Thu, Jan 17, 2019 at 05:09:44PM +0100, Johan Hovold wrote:
> On Tue, Jan 15, 2019 at 03:46:28PM -0800, Matthias Kaehlcke wrote:
> 
> > Using RTS seems|ed like a nice solutions, since it's the native way to
> > prevent the controller from sending data, instead of doing some custom
> > hack. However Johan seems to be fairly convinced that flow control and
> > manual toggling of RTS can be problematic, and we have seen that
> > disabling flow control has its own problems. Maybe it's time to
> > consider other solutions like the DISCARD_RX flag we discussed
> > earlier. Not that I really liked this custom solution, but in the end
> > it might be a more robust way to address the issue.
> > 
> > Johan/Marcel/others: Do you have any further ideas or input on this?
> 
> I don't see why deasserting RTS wouldn't work, well at least as long as
> the RTS signal is wired correctly.
> 
> My point was simply that calling serdev_device_set_rts() will generally
> not work unless you first disable automatic (i.e. hw-managed) flow
> control using serdev_device_set_flow_control(). The exact behaviour is
> serial-controller dependent, but I assume the driver needs to be
> platform agnostic.

I observed that the qcom_geni_serial driver doesn't raise RTS with
flow control disabled. It seems we have to investigate why that's the
case. I agree that the driver should be platform agnostic.

Cheers

Matthias